Technological advances in wind turbines continue to demonstrate that energy from wind power offers a commercially viable alternative energy source. Improvements in design have allowed increases in the sizes of wind turbines and rotor blades such that increases in energy output have been realized. However, manufacturing costs present challenges to the development of wind energy technology as a competitive alternative energy source. In particular, factors that contribute to manufacturing costs and energy efficiencies of wind turbines include the design and construction of rotor blades.
Increases in rotor blade size have demonstrated increases in energy production. Large commercial wind turbines often include rotor blades with spans of 40 to 45 meters or greater. Energy extracted from wind turbines depends on the area of the circle of the rotor blade sweep or rotor diameter from blade tip to blade tip. In particular, increases in blade length increase the area of the circle of the blade sweep that can result in capturing more wind power and increasing energy output. For instance, the area of the circle of the blade sweep is proportional to the square of the blade length, such that, a 10% increase in rotor blade length can result in an increase of 20% in a wind turbine's energy output.
However, scaling up rotor blade size and, in particular, blade length results in a corresponding increase in blade weight and thickness, as well as an increase in the blade's strength requirements. Blade weight is a key limiting factor in blade design whereby an increase in the blade size causes the blade weight to increase faster than the corresponding increase in turbine energy output. In particular, increases in blade length can result in exponential increases in blade weight by a factor of 2.5 to 3 due to increases in blade mass and area. Consequent manufacturing costs would be proportional to the increased amounts of materials consumed in fabricating larger blades and, therefore, can become disproportionally high relative to realized increases in energy output, causing diminishing returns on investments in larger blade sizes. Technological improvements have helped to mitigate increases in blade weight resulting from increases in blade size. However, blade weight remains a limiting factor with respect to improving turbine energy output and efficiency. Thus, increasing turbine energy production through blade size and specifically through blade length presents the challenges of balancing blade length, weight, strength and manufacturing costs to produce blades that cost-effectively increase energy output.
Aerodynamic performance and efficiencies of rotor blades are also critical to efficient and cost-effective wind energy production. Optimum performance of rotor blades is essentially a compromise in blade design between blade shape and blade strength. An ideal blade defines along its span a relatively narrow and twisted shape to enable effective aerodynamic performance, while being relatively thick near or at the blade root to provide the blade with sufficient strength to withstand aerodynamic loads. Blade designs are typically more bulbous near the blade root to provide a thickness and strength that compensates for the relatively narrow and lightweight span of the blade.
Prior art rotor blades include twist bend coupled or twist-coupled blades having a structure that passively affects aerodynamic loads during operation of a wind turbine. Blade design and construction dictate aerodynamic performance and, in particular, the elastic or bending properties that blades exhibit when subjected to aerodynamic loads and pressure. Specifically, such desirable mechanical properties may be built into blade structures through blade shape or curvature and blade fabrication materials. In general terms, a twist-coupled blade bends and twists in response to aerodynamic loads to adjust passively its pitch angle along its length. The pitch angle adjusts the wind load acting on the blade. Passive pitching slightly, e.g., by few or several degrees, towards a feathered position enables the blade to passively distribute and shed wind loads during operation. Blade design and fabrication materials and construction techniques can facilitate the extent of coupling of the blade's bending moment with its twist rotation and thereby the level of passive pitch control the blade may achieve. High levels of coupling blade bending moment and twist demonstrate reductions in aerodynamic loads, particularly under extreme wind conditions, as well as reductions in fatigue loads throughout the rotor or the wind turbine. In addition, twist bend coupling enables blades to adjust constantly and quickly to wind gusts and rotational effects. As a result, increases in energy output and decreases in fatigue damage of the rotor and wind turbine are possible.
Passive pitching results from, in part, the elastic deformation and twist bend coupling in the structural laminates, composites, or other materials constructing the blade and, in particular, constructing the load-bearing structures of the blade. Such materials serve as passive structural components that affect the dynamic response of the blade and aerodynamic loads acting on the blade. Studies of blade designs suggest that overall load reduction can depend on, among other factors, the amount of coupling of structural materials and their design and manufacturing. In addition, structural materials and their design can affect blade cost, stiffness, weight, and strength, as well as blade fatigue and operational life.
Prior art composite fabrication processes often limit the stiffness, strength and fatigue life of structural blade components, such as I-beams, spar caps, and shear webs, to less than ideal or maximum levels. The available forms of reinforcing fibers limit improvements in these processes. For example, glass fiber is commonly supplied as a dry or unfilled fabric, a roving, or a pre-impregnated fabric. In each case the supplied material is wound onto a spool or roll to facilitate handling and shipping. However, winding a layer of fibrous material of finite thickness onto a roll induces fiber waviness or lack of total fiber collimation in the final part, which may not be removable and which has been shown to reduce compressive strength. A reduction in compressive strength must then be compensated with more material, which fabricates an undesirably larger, heavier, and more costly component.
A heavier component may also require more labor. Fabricating such structural components as, for example, a spar cap with fibrous or fiber, e.g., glass, composites, meshes, fabrics, layers, and other materials, often requires relatively large volumes of such materials to build up a structural component. Due to the nature and design of such fibrous and fiber composites, meshes, fabrics, layers, and materials, spar cap manufacturing can be labor-intensive. For instance, fabricating spar caps, such as in a mold, often requires more than 50 glass fabric layers in order to produce a spar cap that may be on the order of 30 to 50 meters or more in length and may have a thickness, at some points along its length, of about 40 mm or more. Clearly, a lower material efficiency requires that a greater quantity of fabric must be used to make a thicker spar cap, and more labor is required to fill the spar cap mold.
In addition, employing such fiber-reinforced fabric layers and composite materials typically requires application of an appropriate resin to bond fabric layers and composite materials and thereby define the finished spar cap geometry. Current methods and techniques of applying bonding resin include resin infusion and resin injection. Such methods and techniques involve infusing under vacuum or injecting under pressure a volume of bonding resin into, for instance, a stack of reinforcing fiber fabric layers and subsequently curing the resin to bond the layers. Because of the lack of structural shape inherent in a fiber fabric, such prior art materials are susceptible to fiber wash during resin infusion or injection that results in undesirable wrinkles, buckling, misplacement, and misorientation of fabric layers and composites in the resulting spar cap.
Further, prior art fiber-reinforced fabric layers and composite materials are susceptible to shrinkage during curing of bonding resin. During resin cure, bonding resin may shrink substantially such that resin shrinkage may cause undesirable wrinkling, kinking, and buckling of fabric layers and composite materials during fabrication of structural components.
Again, the forms of the prior art materials do not support a tightly controlled fabricating or molding process, in turn requiring additional material usage. Consequently, fabrication techniques and processes using fiber-reinforced or fibrous materials can affect the overall manufacturing time and cost of turbine blade production.
Thus, it is desirable to form load bearing and structural components of turbine rotor blades using improved fabrication materials and techniques that accommodate increases in rotor blade size, while decreasing blade weight and increasing blade strength. In addition, it is desirable to employ fabrication materials and techniques to produce such load-bearing structures as spar caps to improve rotor blade aerodynamics and, in cases of swept-shaped rotor blades, to contour such structures to conform to blade curvature or sweep. It is also desirable to use fabrication materials and techniques that help to avoid or to minimize fiber wash and wrinkling, while increasing the fiber volume fraction to ensure sufficient strength in such structural components as spar caps. It is also desirable to use fabrication materials and techniques that increase the overall efficiencies of blade manufacturing.
Further, it is desirable to use fabrication materials and techniques that reduce the manufacturing costs and time associated with producing blade components in terms of reducing materials and labor costs, increasing resin infusion/injection rates, and lowering resin cure times.